Neighbour vandalising cars in common parking spaces

I live in an estate with a parking space outside each house and a couple more spaces in front of my house facing a green. These spaces aren't owned by any one house but are common for the line of houses nearest to them, and are used on a first come first serve basis. Our estate is very near Galway city center and so a lot of chancers who don't live here try and take one of these spaces and then head off into town shopping for the day (despite the no parking signs). One of the neighbours seemingly has taken it upon himself to act as "guardian" of these spaces and now writes "ON PARKING" (thats not a typo, he really writes "on"!) in permanent marker all over any cars parked in these spaces that he suspects don't belong there. While I agree with the sentiment I don't agree with his methods (he should call the clampers or something) and the problem now is he's done this to the cars of friends of mine who were visiting my house at the time (for short periods of time). Am I right in saying that what he is doing is criminal damage and illegal? I haven't caught him in the act but am 100% I know who did it, just don't know which house he lives in so haven't yet confronted him about it. Any advice on what I should do here? Report him to the guards?